import { describe, expect, it } from 'vitest'
import { nearestIndex, scrollEdges } from './geometry'

describe('nearestIndex', () => {
  it('returns the index of the offset closest to the scroll position', () => {
    const offsets = [0, 100, 200, 300, 400]
    expect(nearestIndex(offsets, 0)).toBe(0)
    expect(nearestIndex(offsets, 190)).toBe(2)
    expect(nearestIndex(offsets, 260)).toBe(3)
  })

  it('tracks the left-most visible card after paging forward', () => {
    // 10 cards, 100px apart; scrolled two pages of 4 → left edge at card 4.
    const offsets = Array.from({ length: 10 }, (_, i) => i * 100)
    expect(nearestIndex(offsets, 400)).toBe(4)
  })

  it('resolves the clamped end to the left-most visible card, not the last', () => {
    // 10 cards, ~4.5 visible; at the clamped end the focused card is mid-list,
    // never the final card. This is the drift the fix prevents.
    const offsets = Array.from({ length: 10 }, (_, i) => i * 100)
    expect(nearestIndex(offsets, 560)).toBe(6)
    expect(nearestIndex(offsets, 560)).not.toBe(offsets.length - 1)
  })

  it('breaks ties toward the lower index', () => {
    expect(nearestIndex([0, 100, 200], 50)).toBe(0)
  })

  it('defaults to 0 for an empty list', () => {
    expect(nearestIndex([], 123)).toBe(0)
  })
})

describe('scrollEdges', () => {
  const scrollWidth = 1000
  const clientWidth = 450 // max scroll = 550

  it('reports the start when scrolled to 0', () => {
    expect(scrollEdges(0, scrollWidth, clientWidth)).toEqual({
      atStart: true,
      atEnd: false,
    })
  })

  it('reports the end only when scrolled to the maximum', () => {
    expect(scrollEdges(300, scrollWidth, clientWidth).atEnd).toBe(false)
    expect(scrollEdges(550, scrollWidth, clientWidth)).toEqual({
      atStart: false,
      atEnd: true,
    })
  })

  it('does not report the end early just because the last card is visible', () => {
    // The last card is on-screen from scrollLeft 500, but we are not at the
    // end until scrollLeft reaches its 550 maximum. This is the "one extra
    // click to disable" bug the fix removes.
    expect(scrollEdges(500, scrollWidth, clientWidth).atEnd).toBe(false)
  })

  it('treats a non-overflowing container as both start and end', () => {
    expect(scrollEdges(0, 400, 450)).toEqual({ atStart: true, atEnd: true })
  })
})